The Zener solar engine is, as its name implies, a simple
type 1 solar engine based on a Zener
diode. This is the original solar engine design, by Mark
Tilden, no less!

How it works (simplified)
The capacitor charges until the PNP
transistor (here shown as a 2N3906,
but you could also use a BC327) receives basecurrent
through the Zener
and turns on. Then the NPN
transistor (here shown as a 2N3904,
but you could also use a BC337) turns on and the capacitor
is discharged through the motor. As the NPN
turns on the 2.2K resistor starts to supply basecurrent
to the PNP
and the circuit snaps on. When the capacitor voltage drops
below about 1V, the the PNP
turns off, the NPN
turns off and disconnects the motor from the capacitor which
starts to charge up again.

The voltage across the capacitor rises slowly as it is
charging from the output of a solar cell. This voltage also
appears across the Zener
in series with the PNPbaseemitter
junction. The 2.2 KOhm resistor is connected in series
with the motor to the cap and both are in parallel
with the base
of the PNP.
When the voltage across the Zener
rises above the Zener
voltage, it starts to conduct. Now a trickle of
current
passes through the 2.2K resistor and the motor and until the
current rises to 250uA, the voltage drop at the base
of the PNP
is less than 0.6V. At that point the PNPbase
voltage is high enough for the PNP
to start to turn on. This applies current
to the base
of the NPN
transistor , which then provides a direct motor current
path. As the NPNcollector
voltage drops to 0V, the current
through the 2.2K resistor reverses and starts to
supply the basecurrent
for the PNP,
taking the Zener
diode essentially out of the circuit. The motor draws
current
until the voltage on the storage capacitor is down to about
1V.

Note that you can replace the Zener
diode with one or two diodes in series (trip voltage = 0.5V
times number of diodes), or with LEDs
in series (trip voltage = 1.4V times number of LEDs):

Wilf Rigter's comments on this SE:
The problem with this simple SE design is that it only works
with just the right components, the most important of these
being the motor. If the motor is too big or inefficient it
will not work at all. If the motor is just slightly out of
range of the required parameter you may be able to get it to
work here by replacing the 2.2K resistor with a 10K pot and
adjusting it to get reliable operation. Once set up the pot
can be replaced with an equal fixed value resistor. The
circuit as shown generally works OK with a 30 Ohm motor over
a narrow range of light conditions.

Wilf also provided a much more detailed explanation of
just how this SE design functions:

Without feedback these simple SE circuits would really be
simple. But with feedback and despite the simple design, the
SE circuit operation is really quite complex. Let's walk
through the circuit and build up a mental model of the
operation of the Zener
SE circuit to understand just what those electrons are up
to. There are two kinds of feedback, negative and positive
in the SE circuit and as you might guess, they tend to fight
each other. Let's change the circuit slightly to separate
the two kinds of feedback and discuss them one at a
time.

NEGATIVE FEEDBACK

Assume the 2.2K resistor is not in the circuit (i.e. no
connection):

As long as both transistors are off the cap
keeps charging up from the current of the solar cell
toward the maximum available voltage. When the voltage on
the cap rises to the reverse breakdown voltage of the
Zener in series with the PNPbase
/ emitter
junction, then the PNP
"starts" to come on.

Now without the 2.2K resistor the circuit will sit on
the delicate balance of negative feedback.

The PNPcollector
amplifies the base
current from the Zener
by about 50 times. This 50x amplified Zener current
starts to flow in the NPNbase
which then amplifies the Zener
current in the NPNcollector
by about 50 times for a total amplification gain from
PNPbase
current to motor of 50 x 50 = 2500. When the NPN
turns on it "starts" to discharge the capacitor; as this
happens, the voltage on the capacitor drops, which lowers
the Zener current, which lowers the PNPcollector
current, which lowers the NPNcollector
current. This is the stabilizing or balancing effect of
negative feedback. In fact, the circuit smoothly reaches
equilibrium when the NPN
comes on just enough to dump 98% of the short-circuit
current (Isc) of the solar cell; the remaining 2% flows
through the PNP
and Zener.

So without the 2.2K resistor, the effect of the Zener
conduction would stop there: the NPNcollector
current through the motor would be exactly equal to Isc.
If that Isc is less than the minimum current required to
turn the motor, nothing else happens except for the small
voltage drop across the stalled motor winding from the
NPNcollector
current. This circuit is used in other applications as a
Shunt Voltage Regulator, as it regulates the voltage
across the cap.

POSITIVE FEEDBACK

With the 2.2K resistor connected to the NPNcollector
the circuit behavior is more complicated and first we look
at how it affects the SE triggering process.

When the Zener
starts to turn on the Zener
current flows through the 2.2 K resistor and the motor
winding and generates a voltage at the base
of the NPN
equal to V = I x R (ohm's law). But no current can flow
through the PNPbase
until the base
/ emitter
voltage is about 0.55 V. That means you need a minimum
current of I = E / R or about 0.25 mA through the 2.2 K
resistor before any current even starts to flow in the
PNPbase.

If the solar cell can't deliver this 2.5 mA current
through the 2.2 K resistor, the circuit operation stalls
there. This problem is often traceable to the fact that
the current that the solar cell can deliver drops off
rapidly when the voltage approaches the maximum solar
cell voltage. So the solar cell must have both the right
current and voltage for a specific SE. A larger resistor
(i.e. 10 KOhm) may help here but at a price as noted
later. An alternative solution is to add a diode in
series with the 2.2 K resistor with the anode connected
to the NPNcollector.
Yet another way to improve the initial turn on is to add
a small (.22 uF) cap across the 2.2 K resistor to amplify
the switching voltage without interfering with the DC
characteristics.

If the solar cell can generate enough current to
overcome the first obstacle of the voltage drop across
the 2.2 K resistor, the PNP
starts to turn on and provide base
current for the NPN
and the NPNcollector
current starts flowing through the motor winding which
causes a voltage drop.

Now comes the magic of positive feedback.

When the NPNcollector
voltage starts to drop from the voltage across the motor
winding, that reduces the voltage and current in the 2.2
K resistor which causes more of the Zener
current to flow into the PNPbase
which increases the NPN
current and lowers the collector
voltage even more. This causes a rapid escalation in
current flowing through NPN
and the motor winding. All of this occurs rapidly but is
not instantaneously since the switching process is a race
between positive feedback and negative feedback, as the
additional current in the NPN
causes a voltage drop in the photocell voltage which then
reduces the Zener
current, etc.

So we need a bit more than just 0.25 mA to overcome
the negative feedback part. So if things don't stall in
the previous step, the positive feedback takes over as
the NPNcollector
voltage drops below the Zener
voltage, the voltage and current in the 2.2 K resistor
actually reverse (so instead of draining current away
from the PNPbase
it starts to supply extra PNPbase
current). This results in even more NPN
current but also starts to drop the voltage on the
capacitor since it now needs to supply most of the
current flowing through the motor.

You can see now the importance of a capacitor that has
a low internal resistance and can supply current quickly
without much voltage drop.

It is also important that there is sufficient
NPNbase
current supplied to cause the NPNcollector
voltage to drop to saturation (low voltage drop) and
operate in the nonlinear region. This makes the SE more
efficient and reduces instability during triggering. This
means that the 2.2 K resistor must be optimized depending
on the gain of the transistors and the motor load
current. A 10 K potentiometer can be used to adjust this
base
current for best operation, being replaced with a fixed
resistor after the correct resistance is found.

Since the motor is an inductor and inductors are
electromagnetic devices which resist rapid change in
current, this helps speed up the positive feedback and
switching of the SE, since the voltage on the NPNcollector
can drop rapidly without an instantaneous change in
collector
current.

If all is well and the positive feedback won the race,
the SE is "latched on" like an SCR until the voltage on
the capacitor drops below two base
/ emitter
voltages (1.2 V) at which point the PNP
and NPNbase
currents approach zero.

Positive feedback is also required to successfully
reset the SE and start a new cycle and this is the second
obstacle that a successful SE design must overcome.
Similar to the race between positive and negative
feedback above, it is important that the transistors turn
off rapidly but several factors tend to prevent this.

Ironically, if the solar cell short circuit current is
high there may be enough base
current to keep the transistors on and prevent the SE
from resetting. The symptoms are initial voltage rise on
the cap, then trigger and then the voltage on the cap
remains below the trigger voltage until the charging
current is interrupted, i.e. by blocking the light.

When the transistor base
currents drop the NPN
will come out of saturation
and the rising collector
voltage will reduce the current through the 2.2 K
resistor which turns off the PNP
which turns off the NPN.
A cap across the motor can reduce the rate at which this
happens, and interfere with the SE reset.

One more comment -- sometimes a capacitor is placed in
parallel with the motor winding. Remember that an inductor
in parallel with a capacitor forms a resonant circuit in
which at one frequency the signal losses approach zero and
small voltage oscillations build up to become large
oscillations. Add some transistors and feedback and there is
a tendency for the circuit operation to stabilize around
this particular instability, generating acoustic noise and
vibration in the motor windings instead of motor
rotation.

It should be pointed out that stray capacitances have the
same effect and that as long as the oscillation is short
duration and leads to SE trigger or reset it can be
advantageous to the circuit operation. Although Zener
type SE circuits have problems with capacitors across the
motor winding, FLED SE
circuits seem to prefer it and 1381-based
SEs [see the 1381-based,
"Miller," and VTSE
designs -- Ed.] generally don't care.